LED drive apparatus, systems and methods

ABSTRACT

For controlling a level of luminance produced by a color light-emitting diode (LED) array, a method includes: for a predetermined flux bit-slice period, activating a color enable signal to select a primary color LED and to select a predetermined light flux magnitude set-point; selectively charging an energy storage device and discharging the energy storage device through the selected primary color LED to generate a light flux output during the flux bit-slice period; adjusting a rate of selectively charging the energy storage device to maintain a magnitude of the light flux output at the predetermined light flux magnitude set-point during the flux bit-slice period; and adjusting the predetermined light flux magnitude set-point over the life of the selected LED as the selected LED ages as a function of an anode-to-cathode voltage drop across the selected LED for a given magnitude of current flowing through the selected LED.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/030,746 filed Sep. 18, 2013, which claims the benefit of priority toU.S. Provisional Patent Application Ser. No. 61/704,131 filed Sep. 21,2012, both of which are hereby fully incorporated herein by referencefor all purposes.

TECHNICAL FIELD

This relates generally to light-emitting diodes (LEDs), and moreparticularly to LED luminance control suitable for high dynamic rangeambient light environments.

BACKGROUND

Automotive LED application designs, such as LED illuminatedmicro-display console systems and heads-up display (HUD) systems, facechallenging requirements. These include extended operating temperaturerange, very wide dimming ratio (ratio of fullest brightness image forfull sunlight to lowest brightness image for night darkness), andtight/high quality color point control requirements.

Typical solutions use LED current as the primary feedback mechanism.Some solutions perform dimming by decreasing either amplitude orduration of current through one or more LEDs.

Texas Instruments DLP® DMD projection technology is a mature technologywidely used in numerous display applications, hand held projectors,conference rooms, and digital theaters.

SUMMARY

In described examples for controlling a level of luminance produced by acolor light-emitting diode (LED) array, a method includes: for apredetermined flux bit-slice period, activating a color enable signal toselect a primary color LED and to select a predetermined light fluxmagnitude set-point; selectively charging an energy storage device anddischarging the energy storage device through the selected primary colorLED to generate a light flux output during the flux bit-slice period;adjusting a rate of selectively charging the energy storage device tomaintain a magnitude of the light flux output at the predetermined lightflux magnitude set-point during the flux bit-slice period; and adjustingthe predetermined light flux magnitude set-point over the life of theselected LED as the selected LED ages as a function of ananode-to-cathode voltage drop across the selected LED for a givenmagnitude of current flowing through the selected LED.

In other examples for controlling a level of luminance produced by acolor light-emitting diode (LED) array, a method includes: for apredetermined flux bit-slice period, activating a color enable signal toselect a primary color LED and a predetermined light flux pulsemagnitude set-point; establishing a number of light flux pulses to begenerated during the flux bit-slice period; during the flux bit-sliceperiod and for each light flux pulse, discharging current from an energystorage device into the selected LED; during the flux bit-slice periodand for each light flux pulse, bypassing current from the selected LEDand recharging the energy storage device when a sensed light fluxmagnitude reaches the light flux magnitude set-point; and adjusting thepredetermined light flux pulse magnitude set-point and/or the number oflight flux pulses to be generated during the flux bit-slice period overthe life of the selected LED as the selected LED ages as a function ofan anode-to-cathode voltage drop across the selected LED for a givenmagnitude of current flowing through the selected LED.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art diagram illustrating luminance control of a singleLED using a three-bit binary word to “bit-weight” a stream of LED drivecurrent pulses.

FIG. 2A is a prior art diagram illustrating a stream of LED drivecurrent pulses created by combining multiple, differently bit-weightedsub-streams of LED drive current pulses, each sub-stream correspondingto a particular primary color.

FIG. 2B is a prior art diagram illustrating various lengths or “bitslices” of LED drive current pulses combined to create an examplecomposite drive current signal to drive a color LED array.

FIG. 3 is a prior art diagram illustrating luminance over time of eachprimary color of a white-balanced output of a color LED array at fullbrightness.

FIG. 4 is a prior art diagram illustrating luminance over time of eachprimary color of a white-balanced output of a color LED array dimmed to50% of full brightness by attenuating current flow through the LEDs.

FIG. 5 is a diagram illustrating timing of current pulses associatedwith each primary color of a color LED array dimmed to 10% of fullbrightness by time-attenuating each bit slice according to exampleembodiments.

FIG. 6 is a diagram illustrating luminance over time of each primarycolor of a white-balanced output of a color LED array dimmed to 2.5% offull brightness using both current attenuation and time attenuationaccording to example embodiments.

FIG. 7 is a diagram illustrating luminance over a single bit-slice timeperiod of an LED operating in continuous mode according to exampleembodiments.

FIG. 8 is a diagram illustrating luminance pulses from an LED operatingin discontinuous mode over a bit-slice period according to exampleembodiments.

FIG. 9 is a diagram illustrating luminous pulses over a bit-slice periodof an LED operating in discontinuous mode for example dimming ratiosachievable by varying the number and amplitude of pulses according toexample embodiments.

FIGS. 10A, 10B and 10C are a flow diagram illustrating a method ofcontrolling a level of luminance produced by a color LED array accordingto example sequences.

FIG. 11 is a timing diagram illustrating timing associated withcontinuous-mode operation according to the example sequences illustratedby the method of FIGS. 10A, 10B and 10C.

FIGS. 12A, 12B and 12C are a flow diagram illustrating a method ofdimming a color LED array in discontinuous-mode operation according toexample sequences.

FIG. 13A is a timing diagram illustrating timing associated withdiscontinuous-mode operation according to the example sequencesillustrated by the method of FIGS. 12A, 12B and 12C.

FIG. 13B is a timing diagram with an expanded time axis illustratingtiming associated with discontinuous-mode operation according to theexample sequences illustrated by the method of FIGS. 12A, 12B and 12C.

FIGS. 14A and 14B are a schematic diagram illustrating an apparatus forcontrolling levels of luminance produced by a color LED array accordingto example embodiments.

FIG. 15 is a system diagram illustrating an example head-up displaysystem using apparatus for controlling levels of luminance produced by acolor LED array according to example embodiments.

DETAILED DESCRIPTION

FIG. 1 is a prior art diagram illustrating luminance control of a singleLED using a three-bit binary word to “bit-weight” a stream of LED drivecurrent pulses. Accordingly, the size of the binary control worddetermines the average number of current pulses per time applied to theLED. And, the resulting intensity of light emanating from the LED is afunction of the average number of current pulses per time. In thisdescription, the term “bit-slice” means a period of time (e.g., theperiod of time 110) during which one or more pulses of current areapplied to an LED.

FIG. 2A is a prior art diagram illustrating a stream of LED drivecurrent pulses created by combining multiple, differently bit-weightedsub-streams of LED drive current pulses, each sub-stream correspondingto a particular primary color.

FIG. 2B is a prior art diagram illustrating various lengths or “bitslices” of LED drive current pulses combined to create an examplecomposite drive current signal 212 to drive a color LED array. In thisexample, current pulses of equal bit-slice length are created for eachprimary color. Such balancing in the temporal domain may be done tocreate a net white point in the color domain. However, the bit-slicelengths for a particular group of primary colors vary over time. Thelatter technique may assist in the visual integration of an image by thehuman eye to avoid the appearance of flicker, for example.

FIG. 3 is a prior art diagram illustrating luminance over time of eachprimary color of a white-balanced output of a color LED array at fullbrightness. Such luminous output may result from the current drivesignal 212 of FIG. 2B. The luminous intensity of each primary color isdifferent in this example. Such magnitude differences may be implementedto maintain a given white point and avoid a color cast, given theequivalence of bit-slice periods for each primary color.

FIG. 4 is a prior art diagram illustrating luminance over time of eachprimary color of a white-balanced output of a color LED array dimmed to50% of full brightness by attenuating current flow through the LEDs.Dimming via current flow attenuation is a traditional means of dimming,but is insufficient for high dimming ratios for at least the reasonsmentioned hereinabove.

FIG. 5 is a diagram illustrating timing of current pulses associatedwith each primary color of a color LED array dimmed to 10% of fullbrightness by time-attenuating each bit slice according to exampleembodiments. Current is turned on for a selected portion of each bitslice period, resulting in a dimming ratio that is a function of theon-time.

FIG. 6 is a diagram illustrating luminance over time of each primarycolor of a white-balanced output of a color LED array dimmed to 2.5% offull brightness using both current attenuation and time attenuationaccording to example embodiments. Some embodiments operate in a mannerreferred to herein as continuous mode (“CM”). CM operation includescontrolling both the magnitude of current through a selected LED and theon-time of the LED as a percentage of the bit-slice period. The netdimming ratio is a function of the mathematical product of the currentattenuation and the time attenuation. In the example of FIG. 6, theattenuation is (0.10)*(0.25) or 2.5%.

FIG. 7 is a diagram illustrating luminance over a single bit-slice timeperiod of an LED operating in CM according to example embodiments. FIG.7 illustrates a dimming ratio of 32:1 accomplished by limiting lightflux magnitude to 25% of maximum available amplitude and limiting lightflux pulse width to 12.5% of the bit-slice period, resulting in anattenuation factor of (25%)*(12.5%)=0.03125 or 32:1. Embodiments hereinuse CM operation in high ambient light situations when lower dimmingratios are appropriate.

FIG. 8 is a diagram illustrating luminance pulses from an LED operatingin a manner referred to herein as discontinuous mode (“DM”) over abit-slice period according to example embodiments. During DM operation,multiple light flux pulses of a selected magnitude are generated.

FIG. 9 is a diagram illustrating luminous pulses over a bit-slice periodof an LED operating in DM for example dimming ratios according toexample embodiments. A flux magnitude feedback loop is used to veryaccurately control flux pulse magnitude such that extremely small pulsesmay be created, as further described hereinbelow. DM operation is usedby embodiments herein in low ambient light conditions when very largedimming ratios are appropriate. The two examples 910 illustrates thatthe dimming ratio (e.g., in this example 20:1) may be controlled via thenumber and/or sizes of the multiple light flux pulses.

FIGS. 10A, 10B, and 10C are a flow diagram illustrating a method 1000 ofcontrolling a level of luminance produced by a color LED array accordingto example sequences. FIG. 11 is a timing diagram illustrating timingassociated with CM operation according to the example sequencesillustrated by the method of FIGS. 10A, 10B and 10C. The followingdescription of the method 1000 references the timing diagram of FIG. 11to describe control methods associated with CM operation.

The method 1000 includes selectively charging an energy storage deviceand discharging the energy storage device through the selected primarycolor LED to generate a light flux output during the flux bit-sliceperiod (e.g., the period 1108 illustrated by the dashed-line waveform1110 of FIG. 11). The method 1000 also includes adjusting a rate ofselectively charging the energy storage device to maintain a magnitudeof the light flux output at the predetermined light flux magnitudeset-point (e.g., the set-point 1115) of FIG. 11) during the fluxbit-slice period. In some versions, the method 1000 further includesadjusting the predetermined light flux magnitude set-point over the lifeof the selected LED as the selected LED ages. The amount of aging andcorresponding adjustment in set-point is a function of theanode-to-cathode voltage drop across the selected LED for a givenmagnitude of current through the selected LED.

The method 1000 commences at block 1010 with selecting a primary colorfor the current bit-slice period and continues at block 1014 withselecting a maximum LED current threshold value. The method 1000 alsoincludes setting a pulse period timer value associated with the fluxbit-slice period 1108, at block 1016. The method 1000 further includesactivating a current drive enable signal (e.g., the signal “D_EN” 1118of FIG. 11) to begin charging the energy storage device, at block 1020.

The method 1000 continues at block 1023 with activating a color enablesignal (e.g., the “G_EN” signal 1124 of FIG. 11) associated with theselected primary color. The method 1000 includes selecting apredetermined light flux magnitude set-point control signal (e.g., thecontrol signal 1110) associated with the selected primary color usingthe color enable signal (e.g., the color enable signal 1124), at block1026. The method 1000 also includes enabling a pass transistorcorresponding to an LED of the selected primary color using the colorenable signal, at block 1028.

The method 1000 continues at block 1031 with disabling a current bypassswitch used to shunt current away from the LED array. In some versionsof the method 1000, the latter operation may be accomplished bytransitioning a “shunt enable signal” (e.g., the “S_EN” signal of FIG.11) to a low state. The method 1000 includes sensing the magnitude ofthe light flux output from the selected LED at a flux sensor andgenerating a corresponding flux level signal (e.g., the signal 1135 ofFIG. 11), at block 1034. The method 1000 also includes comparing thesensed magnitude of the light flux output from the selected LED to thelight flux magnitude set-point, at block 1039. The result of the compareoperation is illustrated by example as the “F_CMP_OUT” signal 1140 ofFIG. 11). The method 1000 further includes determining if the sensedmagnitude of the light flux output is greater than or equal to the lightflux magnitude set-point, at block 1041. If so, the method 1000 includesdisabling a current drive source (e.g., via the signal “CNTRL_OUT” 1145)to the energy storage device, at block 1044, until the sensed magnitudeof the light flux output is less than the light flux magnitude set-pointas determined at block 1041.

If the sensed magnitude of the light flux output is less than the lightflux magnitude set-point, the method 1000 continues at block 1047 withsensing a magnitude of current flowing through the selected primarycolor LED and generating a corresponding current magnitude signal. Themethod 1000 includes comparing the sensed magnitude of current flowingthrough the selected primary color LED to a maximum LED currentthreshold value, at block 1050. The method 1000 also includesdetermining whether the sensed magnitude of current flowing through theselected primary color LED is equal to or greater than the maximum LEDcurrent threshold value, at block 1053. If so, the method 1000 includesdisabling the current drive source to the LED array to limit themagnitude of current flowing through the selected primary color LED tothe maximum LED current threshold value, at block 1044, and continuingto sense the level of the light flux output, at block 1034.

The method 1000 continues at block 1058 with determining whether theflux bit-slice timer has expired. If not, the method 1000 includescontinuing to sense the level of the light flux output, at block 1034.If the flux bit-slice timer value has expired, the method 1000 includesenabling the current bypass switch to sharply terminate current flowthrough the selected LED, at block 1061. Upon flux bit-slice timerexpiration, the method 1000 also includes deactivating the current driveenable signal to disable the LED current source, at block 1064, anddeactivating the primary color enable signal, at block 1067. The latteroperation in turn disables the pass transistor associated with theselected LED and de-selects the flux magnitude set-point signal.

FIGS. 12A, 12B and 12C are a flow diagram illustrating a method 1200 ofdimming a color LED array in discontinuous-mode (DM) operation accordingto example sequences. FIG. 13A is a timing diagram illustrating timingassociated with DM operation according to the example sequencesillustrated by the method of FIGS. 12A, 12B and 12C. FIG. 13B is atiming diagram with an expanded time axis illustrating timing associatedwith DM operation according to the example sequences illustrated by themethod of FIGS. 12A, 12B and 12C.

The method 1200 includes discharging current from an energy storagedevice into a selected LED during a flux bit-slice period (e.g., theperiod 1308 of FIG. 13A) to create each of one or more light flux pulses(e.g., the four red pulses within the dashed-line waveform 1310). Themethod 1200 also includes bypassing current from the selected LED andrecharging the energy storage device when a sensed light flux magnitude(e.g., the magnitude 1312 of FIGS. 13A and 13B) reaches a light fluxmagnitude set-point control signal amplitude (e.g., the magnitudeset-point amplitude 1315 of FIGS. 13A and 13B) to quickly terminate eachlight flux pulse.

In some versions, the method 1200 may also include making adjustmentsfor LED aging over the life of the selected LED to maintain a consistentwhite point. Such adjustments may include adjusting the predeterminedlight flux pulse magnitude set-point and/or the number of light fluxpulses to be generated during the flux bit-slice period. Aging may bedetermined by measurements taken of the anode-to-cathode voltage dropacross the selected LED for a given magnitude of current flowing throughthe selected LED.

The method 1200 commences at block 1205 with selecting a primary colorfor the current bit-slice period and continues at block 1208 withinitializing a bit-slice pulse counter used to track the number of lightflux pulses. The method 1200 includes establishing a number of lightflux pulses to be generated during the flux bit-slice period, at block1211. The method 1200 also includes establishing a current supplyset-point control signal for the current bit-slice, at block 1212. Themethod 1200 further includes activating a current drive enable signal(e.g., the “D_EN” signal 1318 of FIG. 13A) to begin charging the energystorage device supplying current to the LED array, at block 1213.

The method 1200 also includes activating a color enable signal (e.g.,the “G_EN” signal 1325 of FIG. 13A) associated with the primary color,at block 1215. The color enable signal is used to select the primarycolor LED and the predetermined light flux pulse magnitude set-point forthe predetermined flux bit-slice period.

The method 1200 continues at block 1221 with selecting the flux pulsemagnitude set point control signal (e.g., “PWM_OUT” 1315 of FIGS. 13Aand 13B) associated with the selected primary color for the bit-sliceperiod. The method 1200 also includes enabling a pass transistorcorresponding to the selected LED, at block 1225. The method 1200further includes disabling a current bypass switch to enable currentfrom the energy storage device to the selected LED, at block 1228. Insome versions of the method 1200, a falling edge of a shunt enablesignal (e.g., “S_EN” 1330 of FIGS. 13A and 13B) may be used to disablethe current bypass switch. As bypassed current begins to decrease,forward voltage at the selected LED (e.g., the forward voltage signal1333 of FIG. 13B) begins to increase during the period 1334. Currentbegins to flow through the selected LED and light flux begins to besensed at the point 1335.

The method 1200 also includes maintaining an available current (e.g.,the current 1350 of FIG. 13B) through an energy storage device such asan inductor used to supply current to the LED array. The method 1200thus includes sensing a magnitude of current flowing through the energystorage device and generating a corresponding current magnitude signal,at block 1231. The method 1200 also includes comparing the magnitude ofcurrent flowing through the energy storage device to the current supplyset-point control signal (e.g., the set-point control signal 1355 ofFIG. 13B), at block 1233. The method 1200 further includes determiningwhether the magnitude of current flowing through the energy storagedevice is greater than or equal to the magnitude of the current supplyset-point control signal, at block 1236. If so, the method 1200 includesdisabling the current drive source at block 1238 to decrease current tothe energy storage device until the magnitude of current flowing throughthe energy storage device is less than the magnitude of the currentsupply set-point control signal and then re-enabling the current sourceat block 1241.

The method 1200 continues at block 1244 with sensing the magnitude ofthe light flux output from the selected LED and generating acorresponding flux level signal (e.g., the flux level signal 1360 ofFIG. 13B). The method 1200 includes comparing the magnitude of thesensed flux level signal 1360 to the magnitude of the flux levelset-point control signal 1315, at block 1247. The method 1200 alsoincludes determining whether the sensed magnitude of the light fluxoutput is greater than or equal to the magnitude of the light fluxmagnitude set-point control signal, at block 1250. (See, e.g., the“F_CMP_OUT” signal 1370 of FIG. 13B.) If not, the method 1200 continueswith sensing energy storage device current magnitude at block 1231.

If the sensed magnitude of the light flux output is greater than orequal to the magnitude of the light flux magnitude set-point controlsignal (e.g., at the point 1312 of FIG. 13B), the method 1200 includesre-enabling the current bypass switch to shunt current away from the LEDarray and terminate the light flux output pulse from the selected LED,at block 1254. (See, e.g., the rising edge 1375 of the current bypassswitch enable signal S_EN.) In the latter case, the method 1200 alsoincludes incrementing the bit-slice pulse counter, at block 1258, anddetermining whether a count of the bit slice pulse counter is equal tothe number of light flux pulses to be generated during the bit-sliceperiod, at block 1261. If not, the method 1200 continues at block 1228with generating an additional pulse.

If the count of the bit slice pulse counter is equal to the number oflight flux pulses to be generated during the bit-slice period, themethod 1200 includes deactivating the color enable signal to disable thepass transistor associated with the selected LED and to disable the fluxpulse magnitude set-point signal, at block 1264. The method 1200 thenrepeats at block 1205 with selecting another primary color for a nextbit-slice period.

FIGS. 14A and 14B are a schematic diagram illustrating an apparatus 1400for controlling luminance levels produced by a color LED array accordingto example embodiments. The apparatus 1400 operates in two modes,continuous mode (CM) and discontinuous mode (DM). Operating in CM, theapparatus 1400 is capable of performing the method 1000 describedhereinabove. Operating in DM, the apparatus 1400 is capable ofperforming the method 1200 described hereinabove. Accordingly, somecomponents of the apparatus 1400 operate in a certain way in CM and in adifferent way in DM. Consequently, the apparatus 1400 will be describedtwice, first in the context of CM operation and then in the context ofDM operation.

Referring to FIG. 14B, the apparatus 1400 includes a parallel array ofLEDs 1405. The parallel array of LEDs 1405 includes one or more LEDs1407, 1408, and 1409 corresponding to each of three primary colors(e.g., red, green and blue). The apparatus 1400 also includes a lightflux sensor 1412 flux-coupled to the LED array to sense a magnitude oflight flux output from a selected LED.

Operating in CM, the apparatus 1400 further includes a pulse widthmodulation (PWM) selector 1414 communicatively coupled to the LED array1405. The PWM selector 1414 selects a predetermined light flux magnitudeset-point signal (e.g., PWM1, PWM2, or PWM3) corresponding to apredetermined primary color. The flux magnitude set-point signal PWM_OUTis selected by a color enable signal (e.g., R_EN, G_EN, or B_EN) for apredetermined flux bit-slice period.

The apparatus 1400 also includes a flux comparator 1418 coupled to thePWM selector 1414 and to the light flux sensor 1412. The flux comparator1418 compares the sensed magnitude of light flux output 1419 to thelight flux magnitude set-point signal PWM_OUT appearing at the input1420.

The apparatus 1400 further includes a current drive circuit 1424communicatively coupled to the flux comparator 1418. The current drivecircuit 1424 selectively charges an energy storage device 1426 (e.g., aninductor) coupled between the current drive circuit 1424 and a commonanode terminal 1428 of the LED array 1405. The energy storage device1426 supplies current to the LED array 1405 when the sensed magnitude oflight flux output is less than the light flux magnitude set-point.

The apparatus 1400 also includes a primary color pass transistor (e.g.,the Red color pass transistor 1431) coupled in series with each primarycolor LED. Each pass transistor is capable of being enabled using aprimary color enable signal (e.g., the R_EN signal 1432) to select anassociated primary color LED (e.g., the Red LED 1407 in this example).

The apparatus 1400 further includes a current bypass switch 1435 coupledbetween an output 1428 of the energy storage device 1426 and a resistor1437 to ground. The current bypass switch 1435 provides fast LED turn-onand turn-off times by selectively shunting current away from the LEDarray 1405.

The apparatus 1400 also includes a current control logic module 1440coupled to the current drive circuit 1424. The current control logicmodule 1440 enables the current drive circuit 1424 during the bit-sliceperiod when the sensed magnitude of light flux output (e.g., the signalappearing at the input 1419 of the flux comparator 1418) is less thanthe light flux magnitude set-point (e.g., the signal appearing at theinput 1420 of the flux comparator 1418 and no over-current conditionexists.

The apparatus 1400 further includes a current comparator 1443 coupled tothe current control logic module 1440. A signal created by the voltagedrop across the resistor 1437 is representative of the magnitude ofcurrent flowing through the selected primary color LED and appears at aninput 1445 of the current comparator 1443. The current comparator 1443compares the latter signal to a predetermined maximum LED currentthreshold signal “C_SET” appearing at an input 1446 of the currentcomparator 1443. The output “C_CMP” of the current comparator 1443toggles the current control logic to maintain the magnitude of currentflowing through the selected primary color LED at or below thepredetermined maximum value of the LED current threshold signal “C_SET.”

Referring to FIG. 14A, the apparatus 1400 also includes a CM logicmodule 1450 communicatively coupled to the current control logic module1440 of FIG. 14B. The CM logic module 1450 provides a drive enablesignal “D_EN” for precise turn-on and turn-off of the selected LED(e.g., the Red LED 1407 when R_EN is active). The CM logic module 1450also provides a shunt enable signal “S_EN” to control the current bypassswitch 1435. The apparatus 1400 further includes a master control logicmodule 1455 coupled to the CM logic module 1450. The master controllogic module 1455 initiates a sequence of flux bit slices and generatesthe set of primary color enable signals R_EN, G_EN, and B_EN. The S_ENsignal is selected from the CM logic module 1450 by a selector 1452 whena DM signal 1456 from the master control logic module 1455 is inactive.

The apparatus 1400 also includes a PWM logic module 1458 coupled to thePWM selector 1414. The PWM logic module 1458 generates the predeterminedlight flux magnitude set-point signals associated with the predeterminedprimary colors for the predetermined flux bit-slice periods. The PWMlogic module 1458 also generates the predetermined maximum LED currentthreshold signal “C_SET.”

The apparatus 1400 further includes an LED aging compensation logicmodule 1462 coupled to the PWM logic module 1458. The LED agingcompensation logic module 1462 monitors the anode-to-cathode voltagedrop across the selected LED for a given current flowing through theselected LED to determine how the LED characteristic curve ages overtime. The LED aging compensation logic module 1462 then adjusts thepredetermined light flux magnitude set-point during the life of theselected LED as the selected LED ages. The apparatus 1400 includes ahigh-side voltage analog-to-digital converter (ADC) 1464 coupled to theLED aging compensation logic module 1462. The high-side voltage ADCconverts a sensed anode voltage of the selected LED to a digital signalfor analysis by the LED aging compensation logic module 1462. Theapparatus 1400 also includes a low-side voltage and current ADC 1466coupled to the LED aging compensation logic module 1462. The low-sidevoltage and current ADC 1466 converts a sensed cathode voltage of theselected LED to a digital signal for analysis by the LED agingcompensation logic module 1462.

The apparatus 1400 will now be described with reference to its structureand operation in DM. Operating in DM, the apparatus 1400 includes theparallel array of LEDs 1405 and the light flux sensor 1412 as describedhereinabove in the context of CM operation. The apparatus 1400 alsoincludes the PWM selector 1414 communicatively coupled to the LED array1405. Operating in DM, the PWM selector 1414 selects a predeterminedlight flux magnitude set-point signal associated with a predeterminedprimary color for a predetermined number of light flux pulses to begenerated during a bit-slice period.

The flux comparator 1418 is coupled to the PWM selector 1414 and to thelight flux sensor 1412 to compare the sensed magnitude of light fluxoutput to the light flux magnitude set-point signal. The current bypassswitch 1435 is coupled between the output 1428 of the energy storagedevice 1426 and the resistor 1437 to ground. The current bypass switch1435 is to be disabled to initiate a ramp-up of LED forward voltage inorder to create a leading edge of a light flux pulse and to be enabledto shunt current away from the selected LED in order to terminate thelight flux pulse when the sensed magnitude of light flux output is equalto or greater than the light flux magnitude set-point, as describedhereinabove with reference to FIG. 13B.

Operating in DM, the apparatus 1400 includes the primary color passtransistors 1407, 1408, and 1409, each coupled in series with a primarycolor LED, each pass transistor capable of being enabled using a primarycolor enable signal to select an associated primary color LED, asdescribed hereinabove with regard to CM operation.

The apparatus 1400 also includes the current comparator 1443communicatively coupled to the LED array 1405. In DM, the currentcomparator 1443 compares the magnitude of current flowing through theenergy storage device 1426 to a predetermined magnitude of current to beregulated through the energy storage device 1426. The current drivecircuit 1424 is communicatively coupled to the energy storage device1426 to selectively charge the energy storage device 1426. The currentcontrol logic module 1440 coupled between the current comparator 1443and the current drive circuit 1424 enables the current drive circuit1424 when the magnitude of current flowing through the energy storagedevice is less than the predetermined magnitude of current to beregulated through the energy storage device.

The apparatus 1400 further includes a DM logic module 1470communicatively coupled to the current bypass switch 1435. The DM logicmodule 1470 selectively enables and disables the current bypass switch1435 via the S_EN signal to control the width of each light flux pulse.(See, e.g., the S_EN waveform 1330 of FIG. 13B.) When the sensed lightflux pulse reaches the flux pulse magnitude set-point 1315 at the point1312, the waveform 1370 of the flux comparator output F_CMP_OUT goeslow. F_CMP_OUT is an input to the DM logic module 1470 and results inS_EN transitioning to a high state. The high state of S_EN turns on thecurrent bypass switch, which abruptly shunts current stored in theenergy storage device away from the selected LED and thus abruptlyterminates the flux pulse.

The master control logic module 1455 is coupled to the DM logic module1470 to establish DM operation when large dimming ratios are desirabledue to low ambient light levels. The master control logic module 1455initiates a sequence of flux bit slices and sequences a set of primarycolor enable signals used to select the predetermined light fluxmagnitude set-point signal associated with the predetermined primarycolor for the predetermined number of light flux pulses. The mastercontrol logic module 1455 also loads a lookup table (LUT) (not shown)located in the DM logic module with the number of pulses to be generatedfor the current and/or subsequent bit-slices. At run-time, the LUTresets the S_EN signal 1330 to initiate each pulse.

The PWM logic module 1458 is coupled to the PWM selector 1414 in DM togenerate the predetermined light flux magnitude set-point signalassociated with the predetermined primary color for the predeterminednumber of light flux pulses to be generated during the bit-slice period.The PWM logic module 1458 also generates a signal C_SET representing thepredetermined magnitude of current to be regulated through the energystorage device.

The LED aging compensation logic module 1462 and associated ADCs 1464and 1466 are structured and operate in DM as described hereinabove inthe context of CM operation.

FIG. 15 is a system diagram illustrating an example HUD system 1500using apparatus for controlling levels of luminance produced by a colorLED array according to example embodiments. The HUD system 1500 includesa parallel array of LEDs 1405 consisting of at least one LEDcorresponding to each of three primary colors. The HUD system 1500 alsoincludes an energy storage device 1426 coupled to the LED array 1405 tosupply current to the LED array 1405. The HUD system 1500 furtherincludes a current bypass switch 1435 coupled to the energy storagedevice 1426. The current bypass switch 1435 shunts current away from aselected LED to provide a fast turn-off time. The HUD system 1500further includes a light flux sensor 1412 flux-coupled to the LED array1405 to sense a magnitude of a light flux output from a selected LED.The HUD system 1500 also includes a primary color pass transistor (e.g.,the pass transistor 1431) coupled in series with each primary color LEDEach pass transistor is capable of being enabled using a primary colorenable signal to select an associated primary color LED.

The HUD system 1500 also includes a CM and DM dimming control module1505. The control module 1505 receives: a light flux sense signal fromlight flux sensor 1412; and a high side voltage sense signal and a lowside voltage sense and current sense signal from the LED array 1405. Thedimming control module 1505 controls current supplied to the energystorage device 1426 and controls the state of the current bypass switch1435 as described hereinabove in the contexts of CM and DM operation.

Accordingly, the dimming control module 1505 includes the pulse widthmodulation (PWM) selector 1414, the flux comparator 1418, the currentdrive circuit 1424, the current comparator 1443, the current controllogic module 1440, the CM logic module 1450, the DM logic module 1470,the master control logic module 1455, the PWM logic module 1458, the LEDaging compensation logic module 1462, the high-side ADC 1464, and thelow-side voltage and current ADC 1466, all coupled together to operateas described hereinabove in the contexts of CM and DM operation.

The HUD system 1500 also includes a digital micro-mirror device (DMD)1510 optically coupled to the LED array 1405. The DMD 1510 includes atwo-dimensional array of pixel-sized mirrors. The mirrors form andproject an image by selectively aiming light flux output from theselected LED into or away from an optics system 1520 on a pixel-by-pixelbasis. The optics system 1520 is also a component of the HUD system 1500and is optically coupled to the LED array 1405 via the DMD 1510. Theoptics system 1520 projects the image formed by the DMD 1510 onto awindshield 1530.

Apparatus, systems and methods described herein may be useful inapplications other than dimming light flux from LED arrays in highcontrast ratio ambient light conditions. Examples of the methods 1000and 1200 and apparatus 1400 for controlling levels of luminance producedby an LED array and the HUD system 1500 provide a general understandingof the sequences of various methods and the structures of variousembodiments. They do not serve as complete descriptions of all elementsand features of methods, apparatus and systems that might use theseexample sequences and structures.

The various embodiments may be incorporated into semiconductor analogand digital circuits for use in receptacle power converters, electroniccircuitry used in computers, communication and signal processingcircuitry, single-processor or multi-processor modules, single ormultiple embedded processors, multi-core processors, data switches, andapplication-specific modules including multi-layer, multi-chip modules,among others. Such apparatus and systems may further be included assub-components within a variety of electronic systems, such astelevisions, cellular telephones, personal computers (e.g., laptopcomputers, desktop computers, handheld computers, tablet computers,etc.), workstations, radios, video players, audio players (e.g., MP3(Motion Picture Experts Group, Audio Layer 3) players), motor vehicles,medical devices (e.g., heart monitor, blood pressure monitor, etc.), settop boxes, and others.

Apparatus and methods described herein provide color LED array dimmingcapabilities applicable to operation in an extremely wide dynamic rangeof ambient light conditions. Light flux levels sensed from a color LEDarray are fed back to control both current availability to the LED arrayand to disable/re-enable a current bypass switch to quickly shuntstored-energy current to or away from a selected LED.

In CM operation, a single flux pulse is created for the duration of thebit-slice period. Feedback from the light flux sensor is used to pulsecurrent to an energy storage device used to supply current to theselected LED such as to maintain the output light flux at apredetermined level or set-point during the bit-slice period. Aparticular dimming level is achieved by establishing both the bit-sliceperiod length and the flux magnitude set-point.

In DM operation, one or more short flux pulses are created during thebit-slice period. Both the turn-on and the turn-off time of each such DMflux pulse are controlled by alternately removing and thenre-establishing a current shunt from the energy storage device toground. Flux pulse magnitude is controlled by recognizing when thesensed flux pulse magnitude has reached a predetermined set-point. Theresulting flux compare signal is used to re-establish the current shuntand to thus abruptly turn off current to the selected LED. A flux pulseof precise amplitude with a sharply falling edge results. Unexpectedlyhigh dimming ratios on the order of 1:4000 or more are achievable in DMoperation.

Accordingly, apparatus, systems and methods described herein implementdynamic dimming of a color LED array, such as may be used in variousapplications operating in high dynamic range ambient light conditions.Such applications may include projection systems of various typesincluding HUD systems, color display panels, outdoor signage, etc.

Embodiments herein may operate in one or both of two modes, “continuousmode” (CM) and “discontinuous mode” (DM). Lower dimming ratios areavailable in CM and higher dimming ratios are available in DM.Consequently, a device or system operating in an extremely wide dynamicrange of ambient light may transition back and forth between CMoperation during periods of high ambient light and DM operation duringperiods of low ambient light. However, methods and structures herein donot so require, because each mode of operation is distinctly supported.

Both modes of operation use light flux levels sensed from the color LEDarray as a feedback signal. The light flux feedback signal is used tocontrol both current availability to the LED array and the state of acurrent bypass switch. The current bypass switch is capable of quicklyshunting stored-energy current to or away from a selected LED. In bothmodes of operation, a target flux level is selected as is a time periodreferred to herein as a “bit-slice” period. One or more single primarycolor LEDs are selected from the array for operation during a singlebit-slice period. For example, for an array with a single LED peradditive primary color, only a single red, green, or blue LED would beselected for operation during a bit-slice period.

In CM operation, a single flux pulse is created for the duration of thebit-slice period. Feedback from the light flux sensor is used to pulsecurrent to an energy storage device used to supply current to theselected LED such as to maintain the output light flux at apredetermined level or set-point during the bit-slice period. Aparticular dimming level is achieved by establishing both the bit-sliceperiod length and the flux magnitude set-point. Dimming ratios on theorder of 1:32 are achievable in CM operation, with the limiting factorbeing unevenness of tracking between LED current and flux output at lowlevels of LED current.

In DM operation, one or more short flux pulses are created during thebit-slice period. Both the turn-on and the turn-off times of each suchDM flux pulse are controlled by alternately removing and thenre-establishing a current shunt from the energy storage device toground. Flux pulse magnitude is controlled by recognizing when thesensed flux pulse magnitude has reached a predetermined set-point. Aresulting flux compare signal is used to re-establish the current shuntand to thus abruptly turn off current to the selected LED. A flux pulseof precise amplitude with a sharply falling edge results. Very highdimming ratios on the order of 1:4000 or more are achievable in DMoperation.

An example automotive and/or aircraft HUD system embodiment is alsodescribed and claimed. In some embodiments, the example HUD system usesTexas Instruments DLP® DMD projection technology in conjunction with theCM and DM dimming apparatus and methods described in detail herein.

Light flux levels sensed from a color LED array control both currentavailability to the array and disable/re-enable a current bypass switchto shunt stored-energy current to or away from a selected LED. Incontinuous mode, a single flux pulse is created for the duration of apre-established period. Feedback from the flux sensor pulses current toan energy storage device to maintain the light flux at a predeterminedset-point. A particular dimming level is achieved by establishing boththe pulse period and the flux magnitude. In discontinuous mode, one ormore short flux pulses are created. Both the turn-on and the turn-offtime of each flux pulse is controlled by alternately removing and thenre-establishing a current shunt from the energy storage device toground. Flux pulse magnitude is controlled by recognizing when the fluxpulse has reached a predetermined set-point and re-establishing thecurrent shunt to abruptly turn off current to the selected LED.

In the drawings, arrows at one or both ends of connecting lines showgeneral directions of electrical current flow, data flow, logic flow,etc., but without limiting such flows to only particular directions(e.g., without precluding such flows in opposite directions).

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. A method of controlling a level of luminanceproduced by a color light-emitting diode (LED) array, the methodcomprising: for a predetermined flux bit-slice period, activating acolor enable signal to select a primary color LED and to select apredetermined light flux magnitude set-point; selectively charging anenergy storage device and discharging the energy storage device throughthe selected primary color LED to generate a light flux output duringthe flux bit-slice period; adjusting a rate of selectively charging theenergy storage device to maintain a magnitude of the light flux outputat the predetermined light flux magnitude set-point during the fluxbit-slice period; and adjusting the predetermined light flux magnitudeset-point over the life of the selected LED as the selected LED ages asa function of an anode-to-cathode voltage drop across the selected LEDfor a given magnitude of current flowing through the selected LED. 2.The method of claim 1, further comprising: selecting a primary color;setting a pulse period timer value associated with the flux bit-sliceperiod; activating a current drive enable signal to charge the energystorage device; enabling a pass transistor corresponding to the selectedLED; and disabling a current bypass switch used to shunt current awayfrom the LED array.
 3. The method of claim 2, further comprising:sensing the magnitude of the light flux output from the selected LED andgenerating a corresponding flux level signal; comparing the sensedmagnitude of the light flux output from the selected LED to the lightflux magnitude set-point; if the sensed magnitude of the light fluxoutput is greater than or equal to the light flux magnitude set-point,disabling a current drive source to the energy storage device until thesensed magnitude of the light flux output is less than the light fluxmagnitude set-point; sensing a magnitude of current flowing through theselected primary color LED and generating a corresponding currentmagnitude signal; comparing the sensed magnitude of current flowingthrough the selected primary color LED to a maximum LED currentthreshold value; and if the sensed magnitude of current flowing throughthe selected primary color LED is equal to or greater than the maximumLED current threshold value, limiting the magnitude of current flowingthrough the selected primary color LED to the maximum LED currentthreshold value and continuing to sense the level of the light fluxoutput.
 4. The method of claim 3, further comprising: determiningwhether flux bit-slice timer has expired and continuing to sense thelevel of the light flux output if the flux bit-slice timer value has notexpired; if the flux bit-slice timer value has expired: enabling thecurrent bypass switch; deactivating the current drive enable signal; anddeactivating the color enable signal.
 5. A method of controlling a levelof luminance produced by a color light-emitting diode (LED) array, themethod comprising: for a predetermined flux bit-slice period, activatinga color enable signal to select a primary color LED and a predeterminedlight flux pulse magnitude set-point; establishing a number of lightflux pulses to be generated during the flux bit-slice period; during theflux bit-slice period and for each light flux pulse, discharging currentfrom an energy storage device into the selected LED; during the fluxbit-slice period and for each light flux pulse, bypassing current fromthe selected LED and recharging the energy storage device when a sensedlight flux magnitude reaches the light flux magnitude set-point; andadjusting the predetermined light flux pulse magnitude set-point and/orthe number of light flux pulses to be generated during the fluxbit-slice period over the life of the selected LED as the selected LEDages as a function of an anode-to-cathode voltage drop across theselected LED for a given magnitude of current flowing through theselected LED.
 6. The method of claim 5, further comprising: initializinga bit-slice pulse counter used to track the number of light flux pulses;selecting a primary color for the flux bit-slice period; establishing acurrent supply set-point control signal for the current bit-slice;activating a current drive enable signal to charge the energy storagedevice; selecting a flux pulse magnitude set point control signalassociated with the selected primary color for the bit-slice period; andenabling a pass transistor corresponding to the selected LED.
 7. Themethod of claim 6, further comprising: disabling a current bypass switchto enable the current from the energy storage device to the selectedLED; sensing the magnitude of the light flux output from the selectedLED; comparing the sensed magnitude of the light flux output to thelight flux magnitude set-point; if the sensed magnitude of the lightflux output is greater than or equal to the light flux magnitudeset-point, re-enabling the current bypass switch to shunt current awayfrom the LED array and terminate the light flux output pulse from theselected LED; incrementing the bit-slice pulse counter; determiningwhether a count of the bit slice pulse counter is equal to the number oflight flux pulses to be generated during the bit-slice period; and ifthe count of the bit slice pulse counter is equal to the number of lightflux pulses to be generated during the bit-slice period, deactivatingthe color enable signal to disable the pass transistor associated withthe selected LED and to disable the flux pulse magnitude set-pointsignal.
 8. The method of claim 7, further comprising: sensing amagnitude of current flowing through the energy storage device andgenerating a current magnitude signal; comparing the magnitude ofcurrent flowing through the energy storage device to the current supplyset-point control signal; determining whether the magnitude of currentflowing through the energy storage device is greater than or equal to amagnitude of the current supply set-point control signal; and if themagnitude of current flowing through the energy storage device isgreater than or equal to the magnitude of the current supply set-pointcontrol signal, de-activating a current supply to decrease current tothe energy storage device until the magnitude of current flowing throughthe energy storage device is less than the magnitude of the currentsupply set-point control signal.